Wood chips for pulp use are usually in the range of about 12 mm to 51 mm in length, and their width is commonly about 12 mm. The grain runs lengthwise of the chips. "Acceptable" chips are normally defined as having a thickness not greater than 8 mm; hence, "over-thick" chips have been defined as these thicker than 8 mm.
In the past over-thick chips have been sliced into thinner pieces by special slicers, destructured by crushing, or split into narrower pieces. In the latter instance, breaking and splitting an over-thick chip along fissures spaced apart across the grain less than 8 mm in effect subdivides the over-thick chip into acceptable chips. This approach to converting over-thick chips to acceptable status is disclosed in U.S. Pat. No. 4,935,795 as an alternative to compressive destructuring between compression rolls having a nip clearance of about 4 or 5 millimeters. In U.S. Pat. No. 4,953,795 chip splitting is disclosed as being performed by oppositely rotating rolls having matrices of pyramid shaped projections formed by machining into the roll surface circumferential v-shaped grooves and axial v-shaped crossing the circumferential grooves at right angles. The rolls are rotatably carried by a frame in a fixed position to define a fixed spacing between the adjacent rolls. The pyramidal projections on the rolls are disclosed as preferably spaced one-half inch apart and having a height substantially equivalent to a desired chip thickness of about 6 mm. The patent mentions positioning the rolls so that the pyramids are in peak to peak orientation, or alternatively, are axially offset into a peak to valley orientation. The patent states that in the latter instance cracks are created in the chips approximately every one-fourth inch when the projections are spaced apart one-half inch and are approximately six millimeters high. This pyramid spacing and height is recommended in the patent as providing desired "aggressively contoured" roll surfaces. However, the patent does not indicate what peak-to-valley spacing between rolls gives a crack spacing of one-fourth inch.
The patent speaks of "mild treatment" and "harsh treatment". For mild treatment the spacing between the pyramid projections in the region where projections from each roll are at their closest is stated to be six millimeters, and for harsh treatment the spacing at the closest point of spacing between projections on separate rolls stated to be three millimeters. The patent does not specifically state whether these dimensions between projections are when the orientation is peak-to-peak or is peak-to-valley, but the peaks between the pyramids in each roll are one-half inch as stated in the patent, then it follows that the projections in each roll can never be closer than one-quarter inch to the projections in the other roll when the orientation is peak-to-valley. Accordingly, the mild treatment and hard treatment examples in the patent appear to be when the orientation is peak-to-peak, or else the harsh treatment to mild treatment spacing range of 3 mm. to 6 mm. is misstated and was intended to refer to the peak to valley distance at the nip with the rolls oriented in a peak to valley relationship. However, the latter arrangement would substantially crush the chips rather than splitting them particularly when the spacing is in the closer part of the spacing range.
Accordingly, U.S. Pat. No. 4,953,795 provides at least one of two oppositely rotating rolls with projections which are aggressively contoured to split over-thick chips in the thickness direction. U.S. Pat. No. 4,953,795 teaches that this is preferred to crushing and destructuring the over-thick chips. The arrangement of the projections in U.S. Pat. No. 4,953,795 is such that normally only the chips approached the nip between the rolls with the chip grain perpendicular to the plane defined by the two roll axes, can be split along the grain in the manner described in the patent. Thus, a relatively large percentage of the chips are not properly oriented for splitting when they pass between the rolls.